Solid-state lighting (SSL) is rapidly becoming commonplace in many areas of modern society. SSL employs light-emitting diodes (LEDs), which are inexpensive, durable, small in size, energy-efficient, and long-lasting. This desirable combination of attributes is not present in more conventional lighting sources, such as incandescent and fluorescent lighting sources.
In some circumstances it is necessary or desirable to control dimming of the light produced by a lighting system. In an SSL system, dimming is typically performed using a technique known as pulse width modulation (PWM). FIG. 1 is a drawing showing the salient components of a PWM-based dimming system 100 typically used to perform dimming in an SSL system. The PWM-based dimming system 100 comprises an LED driver 102, an LED 104 (or string of LEDs connected in series), a transistor 106, and a current-limiting resistor 108. The LED driver 102 generates a PWM control signal which is used to drive the gate of the transistor 106. The PWM control signal comprises a sequence of pulses which determine whether the transistor 106 is turned ON or OFF. Since the transistor 106 is in series with the LED 104, turning the transistor 106 ON and OFF intermittently interrupts the current flowing through the LED 104 and, consequently, intermittently interrupts the light that the LED 104 emits. The durations (i.e., “widths”) of the pulses in the PWM control signal are controlled (i.e., “modulated”) by the LED driver 102, according to the setting of a dim control signal DIM applied to a dim control input of the LED driver 102. As the dim control signal DIM is lowered to effect dimming, the LED driver 102 responds by reducing the widths of the pulses in the PWM control signal. The reductions in the widths of the pulses results in a lower percentage of time during which the LED 104 conducts and emits light. On average, this result is perceived by the human eye as dimming.
In some lighting applications the light produced by a lighting system must be dimmable over a wide dynamic range and to very low light levels. In a movie theater, concert hall, or opera house, for example, it is often desirable to have a dimming dynamic range of 1000:1 or greater. When a PWM-based dimming system is employed to control the dimming, the duty cycle of the PWM control signal must therefore drop to as low as 0.1%, in order to realize the 1000:1 dimming dynamic range. Additionally, the frequency of the PWM control signal must be designed and set high enough to avoid undesirable light flicker. Unfortunately, due to the significant capacitance that is normally present at the input of most commercially available transistors 106, the PWM-based dimming approach is unable to respond properly to a PWM control signal having a duty cycle as low as 0.1% and a frequency that is high enough to avoid flicker, particularly at low light levels. This problem makes PWM-based dimming an unsatisfactory solution to achieving dimming over a wide dynamic range and to very low light levels.
It would be desirable, therefore, to have an SSL dimming system that is capable of realizing dimming over a wide dynamic range and to very low light levels, that is energy efficient, and that does not produce light flicker, especially at low light levels.